The sequence of operations in the manufacture of chemical wood pulp includes debarking and chipping of the wood, pulping or bulk delignification of the chipped or fragmented wood, secondary delignification or bleaching, and subsequent pulp washing, screening and cleaning steps. The present invention is specifically directed to the pulping reaction and delignification steps in which the lignin polymers are separated from the cellulose fibers. Although pulping is fundamental to the manufacture of paper, the pulping reaction itself is not fully understood. Chemical pulping processes are conducted under both (Sulfite Process) and basic (Kraft Process) conditions. The Kraft Process is the dominant technology practiced industrially, accounting for approximately three-fourths of the total pulp production. Kraft Process chemical delignification is conducted by both batch and continuous methods.
The control of such a pulping operation is generally accomplished by means of an open loop, feed forward system. The most common system uses the "H-factor" method. According to the H-factor method, if the initial charge conditions such as chip moisture content, sulfidity, liquor to wood ratio, pH range, and percent active alkali by weight are fixed, then cooking to a given H-factor will result in the same identified pulp yield and lignin content or Kappa number. The time-temperature history of the cook is monitored. The "relative" rate of reaction data for the applicable times and temperatures as set forth by K. E. Vroom, Pulp Paper Mag. Can., Vol 58(3), Page 228 (1957) are generally applicable. Therefore, the relative reaction rate data is integrated over time to predict the degree of delignification associated with the H-factor. The "cook" is then stopped at an H-factor which is known to give the acceptable identified pulp yield. The H-factor method is therefore used to predict when to stop the digestion or "cook" in order to achieve a desired pulp yield. Upon stopping the reaction the pulp must then be analyzed for lignin content to ascertain whether the desired degree of completion has in fact been achieved. These results are used to adjust the cooking parameters to be used on subsequent "cooks".
A disadvantage of the conventional method is that the control of the pulping operation end result is strongly dependent upon the maintenance of uniform conditions of chip moisture content, temperature, chemical feed properties, etc. It has not been possible to measure certain of these variables such as chip moisture content in any practical way. As a result the outcome of one reaction is used to correct the conditions for the next in a trial and error procedure on the assumption that the average chip properties will remain reasonably uniform. Overall, it has not been possible to date to close the open control loop of the H-factor method because of the lack of suitable process measurements.
Another approach to controlling the quality or degree of the pulp delignification is by measuring the "Kappa number". The Kappa number is defined by TAPPI standard T-236, "Kappa Number of Pulp", Technical Association of the Pulp and Paper Industry, Atlanta, Ga. The Kappa number is directly related to the Klason lignin content of the pulp according to a relationship that the percent lignin in the pulp equals 0.147 times the Kappa number. This relationship is described by Casey, Pulp and Paper Chemistry and Chemical Technology, Wiley Interscience, N.Y. (1980) (p. 665). The Kappa number, however, is not suitable for on-line measurement because the pulping reaction must be terminated in order to measure the Kappa number of the residual wood or pulp. Models have been developed for inferring the Kappa number from measurement of other parameters, such as for example, the measurement of sulfidity of the liquor by a selective ion electrode, the conductometric titration of a liquor sample by acid and by optical and calorimetric methods. None of these, however, has been widely accepted by the industry, and virtually all pulping control in the United States is done by means of the H-factor method or equivalent feed forward control. Furthermore, there is no generally accepted procedure for on-line monitoring and control of pulp yield in secondary delignification or bleaching reactions.
U.S. Pat. No. 3,674,434 describes a "method and apparatus for determining lignin content" by direct measurement of a sample "based upon a discovered linear relationship between the lignin content of wood or residual wood pulp and the ratio of elemental carbon to elemental hydrogen in the sample". This method requires that the reaction be terminated for direct sampling of the wood pulp. The sample is dried to a standardized moisture content and then analyzed to determine the ratio of carbon to hydrogen. The lignin content is then computed using an equation which relates percent lignin to the ratio of elemental carbon to elemental hydrogen. This method is unsuited for on-line real time monitoring and process control.
U.S. Pat. No. 4,193,840 describes a method for determining the degree of delignification by monitoring a combination of temperature and pressure. U.S. Pat. No. 4,162,933 seeks to ascertain the degree of delignification by monitoring exothermic heat. Because the measured parameters are only indirectly related to the desired information, the required reliability for on-line process control cannot be achieved.
The Institute of Paper Chemistry (IPC) is currently sponsoring a program for determination of pulp yields in continuous digesters. The technique under investigation by IPC contemplates using the carbohydrate fraction of the pulp as a prediction of yield in continuous digesters. The IPC method depends on the assumption that the yield of cellulose as a percentage of a particular wood species is essentially constant during a Kraft reaction. The IPC method contemplates that accurate determination of pulp carbohydrates by gas chromotography will lead to a prediction of pulp yield. The IPC method, however, is not applicable to on-line real time measurement of pulp yield.